If, Then, Else

A program can choose what to do from the values it holds. Did the player run out of lives? Did the score pass the high score? Did the ship hit the wall? Each of those is a question with two answers, and the program needs to do something different depending on which it is. That’s what If gives you: check something, and branch.

What you’ll see by the end

The program holds 3 lives, checks whether that’s more than zero, and — because it is — prints Still in the game! Had the count been zero, the same program would have printed Game over, man instead. One program, two possible outcomes, chosen as it runs.

The whole program

Hide
Cls 6
LIVES=3
Locate 8,7
Print "Checking your lives..."
If LIVES>0
   Locate 8,10
   Print "Still in the game!"
Else
   Locate 8,10
   Print "Game over, man."
End If
Locate 8,12
Print "Lives left: ";LIVES
Wait Key

The new part is the decision block in the middle:

If LIVES>0
   Print "Still in the game!"
Else
   Print "Game over, man."
End If

Read it as plain English, because that’s almost what it is:

• If LIVES>0 asks a yes-or-no question: is LIVES greater than zero?
• The lines after it run only if the answer is yes.
• Else marks the other path — the lines after Else run only if the answer is no.
• End If closes the block, so AMOS knows where the decision ends and the program carries on.

Exactly one of the two paths runs, never both. AMOS checks the question once, picks the matching branch, and skips the other entirely.

Asking the question

The > in LIVES>0 is a comparison — it’s how you ask AMOS to weigh two values against each other. There’s a small set of them, and they read the way they look:

Each one is a question with a yes-or-no answer, and that answer is what If branches on. (The = here means “is it equal to?”, which is a different job from the = that stores a value, like LIVES=3. AMOS tells them apart by where they sit — after If, it’s a question.)

Indenting the block

Notice the lines inside the If are pushed in a few spaces. AMOS doesn’t need that — it would run the same without it — but the indent shows at a glance which lines belong to the decision and which don’t. Get into the habit now; once programs grow, that shape is what lets you read them.

You don’t always need Else

Sometimes you only want to act when something’s true, and do nothing otherwise. You can leave Else out entirely:

If SCORE>HISCORE
   Print "New high score!"
End If

No Else, so when the score isn’t higher, the block does nothing and the program moves on. If and End If are the essential pair; Else is there for when you need the second path.

Type it and run it

Type the program in and press F1. You’ll see the Still in the game! branch, because LIVES is 3. Press a key to finish the wait, then press Return at the end-of-program message to return to the editor.

Try this: take the other path

Before changing the program, predict which message it will show for LIVES=1, LIVES=0 and LIVES=-1. Explain what > asks. Then change only the assignment and run each case.

Set LIVES=0 and run it again. Now the question LIVES>0 is false, so the Else branch runs and you get Game over, man — and Lives left: 0 below it. Same program, the other outcome, decided entirely by the value you stored.

Try this: a different question

Change the test to If LIVES>=5 and set LIVES=3. Now the answer is no, so the Else branch runs even though there are lives left — because the question changed. The branch you see depends on both the value and what you asked about it.

Try this: find the boundary bug

Set LIVES=0, and change only If LIVES>0 to If LIVES>=0. This is valid AMOS, so a syntax check cannot tell you whether it expresses the rule you want.

If it doesn’t work

• AMOS complains about End If, or runs the wrong lines. This multi-line If needs a matching End If to close it. Leave it off and AMOS doesn’t know where the decision stops.
• Both messages appear, or neither. Check the Else is on its own line, between the two branches — not attached to a Print.
• It complains about the comparison. You’re comparing a number variable here; make sure both sides are numbers. Comparing text needs its quotes, the same as printing it.

What you’ve learnt

Decisions are what let a program react. If asks a yes-or-no question, the lines after it run when the answer is yes, Else gives the path for when it’s no, and End If closes the block. The question is built from a comparison — >, <, =, >=, <=, <> — and exactly one branch runs each time. Drop Else when you only need to act on “yes.”

What’s next

You can make a decision once. Next — Loops — you’ll make the program repeat: do something a set number of times, or keep going until a condition changes. Repetition plus decisions is most of what any game is made of.